docs: def _add_cloud_staging(cloud_archive_release, openstack_release):
      """Add the cloud staging repository which is in
      ppa:ubuntu-cloud-archive/<openstack_release>-staging
  
      This function checks that the cloud_archive_release matches the current
      codename for the distro that charm is being installed on.
  
      :param cloud_archive_release: string, codename for the release.
      :param openstack_release: String, codename for the openstack release.
      :raises: SourceConfigError if the cloud_archive_release doesn't match the
          current version of the os.
      """
      _verify_is_ubuntu_rel(cloud_archive_release, openstack_release)
      ppa = 'ppa:ubuntu-cloud-archive/{}-staging'.format(openstack_release)
      cmd = 'add-apt-repository -y {}'.format(ppa)
      _run_with_retries(cmd.split(' '))